This invention relates to ceramic regenerative heat transfer media for use in Regenerative Thermal Oxidizers.
Conventional ceramic heat-transfer media used in RTOs typically consist of 70xcx9c75% SiO2, 20xcx9c25% Al2O3, 2xcx9c5% K2O or Na2O, and trace amounts of Fe, Ca, and Ti. The media can be shaped like honeycombs, plates, saddles, or other forms. When ceramic media having such composition are used in an alkaline environment, reactions will occur between the surface of the media and the alkaline component.
As a result, a layer of reaction products builds up on the surface of the media, which increases in thickness over time such that the effective void fraction of the media is decreased. The reduction in void fraction eventually reaches a point at which the increased pressure drop of gas flowing through the media impairs the operating efficiency and performance of the equipment, which then has to be shut down to permit replacement of the media. This not only adds to the cost of media, but it may also result in lost production.
Alkali-resistant ceramics used in the metal finishing and glass industries contains silicon carbide, silicon nitride, alumina, zirconia, and similar materials that have to be formed under high pressure. As a result, these ceramics are quite expensive. More importantly, these ceramic compositions cannot be shaped using ordinary methods, so they often cannot be made into the same shapes as conventional ceramic media.
U.S. Pat. No. 5,731,250 to Reid et al. teaches the use of zircon-based ceramic bodies made from a composition that can be formed by conventional processes. However, zircon is expensive, and it would be useful to make heat-exchange media from a material with even better resistance to alkali attack.
As is well known, regenerative thermal oxidizers (RTO) usually consist of two or more heat-exchange canisters with one combustion chamber. Heat-transfer media are installed in the heat-exchange canisters in order to store and release heat.
In the wood process industry, the waste gas treated by RTOs often contains substantial amounts of solid particulates. Analysis by scanning electron microscopy (SEM) indicates that the residue left on the media by partial oxidation of such solid particulates consists of:
Additional analysis using Auger electron spectroscopy (AES) shows that wood ash contains large amounts of Na2SO4, K2CO3, CaCO3, MgO, Al2O3, SiO2 and other inorganic compounds. At the operating temperature of an RTO (850xc2x0 C.), these components can become corrosive. The corrosive nature of these compounds can cause chemical reactions with ordinary porcelain or stoneware. These reactions can reduce the void fraction of the media and impair the performance of the RTO.
Analysis of media that had been installed in an RTO for ten months reveals that the surface layer of the media contained the same components as wood ash, indicating that the wood ash has reacted with that layer of ceramic material.
Analysis by AES and X-ray diffraction spectroscopy shows that the surface layer consists of sodium aluminosilicate (Na2AlSiO4), potassium aluminosilicate (K2AlSiO4), and various other compounds of potassium, calcium, aluminum, silicon and/or sulfur. These are the reaction products that cause problems with the media.
There is therefore a need for an economical alkali-resistant ceramic material that can solve the problem of chemical attack by hot alkali in RTOs.